to do exactly what you're trying to do is TCP 2055 from router to firewall (PAT) to solarwinds. You'd also need that router added as a node with NPM, so SNMP access from Solarwinds to Router.
However, it would be better if you setup netflow on your firewall or internal core switch. The edge router isn't going to see any internal IP's since those are NAT'ed by the firewall. It would basically show your firewall (WAN) as the top talker plus any 1-to-1 NAT's you have.